In a game a group of friends and I are working on using XNA, we wanted to create an effect where we could darken the screen and draw something else on top of the darkened portion. For example, we have a title screen that is drawn as a background sprite, a logo, a few buttons, and a lot of particle effects (all separate Texture2D's), but when you select the "Exit" button, the entire title screen darkens, and two (undarkened) buttons are drawn over-top ("exit" and "cancel" buttons).
Initially, we were going to do this by having two separate render targets, one for the title screen, and one for the yes and no buttons. Then, when we wanted to display the topmost render target, we darkened the render target that the title screen was drawn to, and then draw the new render target on top. The problem with this is that the second render target is filled with black pixels that simply get drawn over the background, blocking it out completely.
We have two ideas to fix this. The first idea is to draw the second render target on top of the first using
BlendState.Additive, so that the black pixels are simply added to the background instead of drawn over-top. The problem with this is that it makes the other buttons look weird, since they're being drawn over top other things on the title screen. The second idea was to manually set every black pixel in the second render target to
Color.Transparent, but we're not sure how efficient that is.
Any ideas on how we could achieve this effect?